Chapter 2
CALCULATIONS REVISION

Calculation Skills for Nurses, Second Edition. Claire Boyd.

© 2022 John Wiley & Sons, Ltd. Published 2022 by John Wiley & Sons Ltd.

LEARNING OUTCOMES

By the end of this chapter you will have familiarised yourself with maths basics and decimals, percentages, fractions, ratios and averages.

FEELING A BIT RUSTY?

Don’t worry if picking up this book and the word ‘calculations’ gave you palpitations! We’ll start nice and gently and summarise the basics. You may remember most of this already and feel confident enough to skip this chapter, and the next completely, or you may wish to build up your confidence and reacquaint yourself with the basics.

Symbols and Signs

Seeing this sign / means divided by…

ADDITION

Maths can be so much easier than you thought as a lot of the time we just need to use our adding skills. For example, a common nursing skill is to tot up a patient’s fluid intake.

Let me show you an example…

Patient A’s total oral intake of fluid has been:

• Water 50 mL
• Tea 150 mL
• Water 150 mL
• Tea 75 mL
• Coffee 50 mL
• Water 75 mL
• Cola 200 mL
• Cocoa 100 mL

All we need to do is add up all these figures:

To begin with, I did a quick rough mental calculation so I knew the answer would be 800 and something. So, 850 mLs looks about right. We will look at Fluid Charts in more detail in Chapter 12.

Sometimes, when we talk ‘maths’, this very word can make us quiver and we put up a brick wall and try to avoid the task in hand. But all we have to do is practice and look at the problem calmly and break the question down.

SUBTRACTION

Let me show you an example…

A baby’s nappy weighs 15 grams when dry. The nappy now weighs 123 grams. How much urine has the baby passed? NOTE: 1 gram = 1 mL.

ANSWER: This is a very simple subtraction question and all we have to do is subtract the weight of the dry nappy from the weight of the wet nappy: 123 grams – 15 grams = 108 mLs.

Have a go at doing one yourself to build your confidence.

Activity 2.1

A baby’s nappy weighs 18 grams when dry. The nappy now weighs 135 grams. How much urine has the baby passed? NOTE: 1 gram = 1 mL.

Don’t over complicate the maths. Break everything down into bite‐size pieces.

It is a good idea to reacquaint yourself with your times tables.

MULTIPLICATION

Next, we have a very basic multiplication question. At first glance you may not notice how basic the question is:

A patient has been prescribed an inhaler due to their Chronic Obstructive Pulmonary Disease (COPD). The inhaler will need to last 28 days. The patient takes three inhalations daily from their inhaler device. How many inhalations does the patient take over the 28 days and will there be enough medication in the device for this period? NOTE: Inhaler contains 200 doses.

ANSWER:

3 = inhalations per day

28 = number of days required

3 × 28 = 84 doses

Therefore, the inhaler device has more than sufficient medication for this time period because it contains 200 doses and only 84 are required for this time period. There is therefore plenty to spare.

DIVISION

We can also work out some basic concentrations of medications in volume using division. Once you have been shown how to do it – it really is very simple! NOTE: we will be looking at slightly more complicated percentage concentrations when working through the percentages section.

Let me show you an example before having a go yourself:

You have 20 mg of a drug in 8 mL of fluid. What is the concentration in mg/mL? To do this, all we have to do is to divide the drug amount with the volume amount:

We can reverse check this: 8 lots of 2.5 = 20 (8 × 2.5 = 20)

Activity 2.2

1. 500 mg Amoxicillin powder is dissolved in 25 mL water for injection. What is the concentration in mg/mL?
2. A syringe contains 20 mg of morphine in 4 mL. What is the concentration in mg/mL?
3. You have 1000 mg of a drug in 20 mL fluid. What is the concentration in mg/mL?

You would have noticed when looking at Question 3 that the names of the drugs did not matter when answering the question as we are looking at the maths aspect only. Sometimes we need to remove the waffle!

DECIMALS

Decimal numbers describe tenths, hundredths and thousandths of a number. For example, 1.25 is equal to one whole unit, plus a fraction of one (25 hundredths). Let me show you this in a visual format:

Decimal

A decimal is a number that is expressed in the counting system that uses units of tens.

Rounding Decimal Numbers

Sometimes it is necessary to ‘round up’ or ‘round down’ a decimal number or a whole number. This is particularly true in infusion drip rate calculations, as it is impossible to give a ‘point’ or...